The article analyzes a copy of a royal rescript (SEG XII. 370 = Rigsby 12 = IG XII. 4. 213) found on Cos. The author of the rescript grants asylia to the festivities dedicated to Asclepius. Comparing the data of the inscription with the materials from the archive of the temple of Asclepius, the author comes to the conclusion that the rescript was issued by Ziaelas and sheds new light on some details of the first half of Ziaelas rule (255–242 BC).